Description: The nature and origin of the J-magnetic anomaly along the Iberia–Newfoundland margins are controversial and its validity for plate kinematic reconstructions questioned. At present, it is interpreted as either an oceanic isochron or an edge effect of oceanic crust corresponding to lithosphere breakup. Both interpretations result in restorations that are in conflict with the current knowledge from Pyrenean and North Atlantic geology. We combine seismic interpretations and dating of magmatic additions with magnetic data to examine the nature and formation process of this anomaly and discuss its value for plate restorations. We show that the J-anomaly is the result of polygenic and multiple magmatic events occurring during and after the formation of the first oceanic crust. Therefore, we conclude that the J-anomaly cannot be used for plate kinematic studies and, more generally, we question the validity of using ill-defined magnetic anomalies outside unequivocal oceanic domains for plate reconstructions.

Description: The breakup of Laurasia to form the Northeast Atlantic Realm disintegrated an inhomogeneous collage of cratons sutured by cross-cutting orogens. Volcanic rifted margins formed that are underlain by magma-inflated, extended continental crust. North of the Greenland-Iceland-Faroe Ridge a new rift–the Aegir Ridge–propagated south along the Caledonian suture. South of the Greenland-Iceland-Faroe Ridge the proto-Reykjanes Ridge propagated north through the North Atlantic Craton along an axis displaced ~150 km to the west of the rift to the north. Both propagators stalled where the confluence of the Nagssugtoqidian and Caledonian orogens formed an ~300-km-wide transverse barrier. Thereafter, the ~150 × 300-km block of continental crust between the rift tips–the Iceland Microcontinent–extended in a distributed, unstable manner along multiple axes of extension. These axes repeatedly migrated or jumped laterally with shearing occurring between them in diffuse transfer zones. This style of deformation continues to the present day in Iceland. It is the surface expression of underlying magma-assisted stretching of ductile continental crust that has flowed from the Iceland Microplate and flanking continental areas to form the lower crust of the Greenland-Iceland-Faroe Ridge. Icelandic-type crust which underlies the Greenland-Iceland-Faroe Ridge is thus not anomalously thick oceanic crust as is often assumed. Upper Icelandic-type crust comprises magma flows and dykes. Lower Icelandic-type crust comprises magma-inflated continental mid- and lower crust. Contemporary magma production in Iceland, equivalent to oceanic layers 2–3, corresponds to Icelandic-type upper crust plus intrusions in the lower crust, and has a total thickness of only 10–15 km. This is much less than the total maximum thickness of 42 km for Icelandic-type crust measured seismically in Iceland. The feasibility of the structure we propose is confirmed by numerical modeling that shows extension of the continental crust can continue for many tens of millions of years by lower-crustal ductile flow. A composition of Icelandic-type lower crust that is largely continental can account for multiple seismic observations along with gravity, bathymetric, topographic, petrological and geochemical data that are inconsistent with a gabbroic composition for Icelandic-type lower crust. It also offers a solution to difficulties in numerical models for melt-production by downward-revising the amount of melt needed. Unstable tectonics on the Greenland-Iceland-Faroe Ridge can account for long-term tectonic disequilibrium on the adjacent rifted margins, the southerly migrating rift propagators that build diachronous chevron ridges of thick crust about the Reykjanes Ridge, and the tectonic decoupling of the oceans to the north and south. A model of complex, discontinuous continental breakup influenced by crustal inhomogeneity that distributes continental material in growing oceans fits other regions including the Davis Strait, the South Atlantic and the West Indian Ocean.

Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q06022, doi:10.1029/2006GC001559.

Description: From 55°45′E to 58°45′E and from 60°30′E to 62°00′E, the ultraslow-spreading Southwest Indian Ridge (SWIR) consists of magmatic spreading segments separated by oblique amagmatic spreading segments, transform faults, and nontransform discontinuities. Off-axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the SWIR. Individual magmatic segments show varying magnitudes and directions of asymmetric spreading, which requires that the shape of the plate boundary has changed significantly over time. In particular, since 26 Ma the Atlantis II transform fault grew by 90 km to reach 199 km, while a 45-km-long transform fault at 56°30′E shrank to become an 11 km offset nontransform discontinuity. Conversely, an oblique amagmatic segment at the center of a first-order spreading segment shows little change in orientation with time. These changes are consistent with the clockwise rotation of two ~450-km-wide first-order spreading segments between the Gallieni and Melville transform faults (52–60°E) to become more orthogonal to spreading. We suggest that suborthogonal first-order spreading segments reflect a stable configuration for mid-ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge-axis that can more easily accommodate seafloor spreading.

Description: Funding for this work came from a JOI-Schlanger Fellowship to Baines and NSF grant 0352054 to Cheadle and John.

Description: The Laccadive–Chagos Ridge and Southern Mascarene Plateau in the north-central and western Indian Ocean, respectively, are thought to be volcanic chains formed above the Réunion mantle plume1 over the past 65.5 million years2,3. Here we use U–Pb dating to analyse the ages of zircon xenocrysts found within young lavas on the island of Mauritius, part of the Southern Mascarene Plateau. We find that the zircons are either Palaeoproterozoic (more than 1,971 million years old) or Neoproterozoic (between 660 and 840 million years old). We propose that the zircons were assimilated from ancient fragments of continental lithosphere beneath Mauritius, and were brought to the surface by plume-related lavas. We use gravity data inversion to map crustal thickness and find that Mauritius forms part of a contiguous block of anomalously thick crust that extends in an arc northwards to the Seychelles. Using plate tectonic reconstructions, we show that Mauritius and the adjacent Mascarene Plateau may overlie a Precambrian microcontinent that we call Mauritia. On the basis of reinterpretation of marine geophysical data4, we propose that Mauritia was separated from Madagascar and fragmented into a ribbon-like configuration by a series of mid-ocean ridge jumps during the opening of the Mascarene ocean basin between 83.5 and 61 million years ago.We suggest that the plume-related magmatic deposits have since covered Mauritia and potentially other continental fragments.

Description: PDF is Published online 23 Feb 2013 version

Description: Published